I'm sorry, this thread has me lost. What is different about this one additional camera that will help the car through an intersection?
The simple answer is field of view.
A future version of Autopilot will likely use a "tri-focal" camera cluster up front. The cameras will be different focal lengths. So, you'll have one that is just looking straight ahead. The left and right edges of the frame are the edges of the road. Then, you'll have another camera with a wide fish-eye lens covering a 180 degree view of the whole forward scene. The wider view will have a lower resolution of the road and vehicles in front of the car, but it will see motion like cross-traffic approaching an intersection. The current camera has tunnel-vision and basically can't turn its head.
Together, a wide angle camera, a road-only camera, and perhaps one in the middle will be able to stitch the scene together. Everything in front of the car will be the highest clarity, able to see a small piece of debris or pothole in the road clearly. The wider cameras will be looking for larger objects, noting their trajectory and also survey for traffic signals and road signs that could be off to the side.
To be honest, I'm surprised we're not hearing more about two cameras placed on each top side corner of the windshield. Having that much separation would allow for the car to "see behind" more objects and counteract glare.